1. Field of the Invention
The present invention generally relates to optical fiber communication systems, and more particularly to a method and an apparatus for the measurement of the distribution of the reflectivity along an optical transmission line to provide the in-service monitoring of a fiber-optic transmission link.
2. Description of the Related Art
The distribution of the back-reflected light along the fiber-optic transmission link is an important parameter for identifying the problems in the outside plant. An optical time-domain reflectometer (OTDR) has been widely used to measure this distribution. In the measurement based on the OTDR, an optical short pulse or OTDR pulse is launched into the fiber-optic transmission link, and the reflected signal is measured as a function of the time like a lidar system. A modern OTDR can provide sufficient spatial resolution and dynamic range required for the characterization of the transmission link. Thus, there have been many efforts to utilize the OTDR for the in-service monitoring of various types of fiber-optic transmission systems. In these techniques, a supervisory channel at a different wavelength from that of transmission signals is used for the OTDR pulses so as not to disturb other signal channels in service.
However, it is not straightforward to apply these techniques based on the OTDR in wavelength-division multiplexed (WDM) networks due to the WDM multiplexers and de-multiplexers placed along the transmission link. For example, in a WDM passive optical networks (WDM PON) implemented by using an arrayed-waveguide grating (AWG) at the remote node (RN), these techniques cannot monitor the failures in the drop fibers (which connects the RN and each subscriber) since the OTDR pulse is blocked at the RN. Several techniques have been proposed to solve this problem by implementing additional couplers to bypass the AWG at the RN (see U. Hilbk, M. Burmeister, B. Hoen, T. Hermes, J. Saniter, and F. J. Westphal, “Selective OTDR measurements at the central office of individual fiber link in a PON,” in Optical Fiber Communication Conference and Exhibit, Technical Digest (Optical Society of America, 1997), paper Tuk3.), using a tunable OTDR (see K. Tanaka, H. Izumita, N. Tomita, and Y. Inoue, “In-service individual line monitoring and a method for compensating for the temperature-dependent channel drift of a WDM-PON containing an AWGR using a 1.6 mm tunable OTDR,” in Proceedings of European Conference on Optical Communication, 3, paper 448, pp. 295-298 (1997)), or generating the OTDR pulse for a specific drop fiber by using the corresponding WDM transmitter (see K. W. Lim, E. S. Son, K. H. Han, and Y. C. Chung, “Fault localization in WDM passive optical network by reusing downstream light sources,” IEEE Photon. Technol. Lett., 17, 2691 (2005); and U.S. Pat. No. 6,548,806). However, it should be noted that all these techniques require the termination of the service of the corresponding WDM channel during the process of monitoring the status of drop fibers. Also, the OTDR using the supervisory channel is not suitable for the use in the conventional PONs. This is because the direct application of these methods has proved to be too costly for the integration with the optical transmitter. In addition, the allocation of the supervisory channel for OTDR prevents the future wavelength assignment for future services, and it can cause transmission impairments caused by nonlinear optical interactions.
One way to cope with these problems is the use of the optical signal light itself for data transmission, instead of using the OTDR pulse. In the digital transmission, each ‘1’ bit is essentially a small OTDR pulse. The reflected signal is a time-delayed superposition of all reflections from all bits that were sent in the past round-trip time. Therefore, by calculating the correlation between the data signal and the back-reflected signal, the reflection point can be found. This idea was implemented in U.S. Pat. No. 4,911,515 by using a variable delay line and a correlator. As is pointed out in W. Soto, “Optical testing for passive optical networks,” Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest (Optical Society of America, 2008), paper NThB5., however, this method is very difficult to apply to a system operated at high data-rate. For example, in a 40-km long optical transmission system, the round-trip time for the reflected light is 400 microseconds at maximum, and it corresponds to 500,000 bits when the data rate is 1.25 Gbit/s. Therefore, for the calculation of the correlation, it is needed to store the data of 500,000 samples at the rate of 1.25 Gsamples/s. This is the large obstacle since the large fast memories and the extremely fast correlator are needed.
In addition, this method has a severe limitation on the dynamic range. This problem was pointed in Y. Takushima and Y. C. Chung, “Optical reflectometry based on correlation detection and its application to the in-service monitoring of WDM passive optical network,” Optics Express, 15, 5318-5326 (2007). The method based on the correlator utilizes the optical signal light which is modulated with the transmission data. The transmission data can be considered a random bit sequence, but the correlation characteristics are not perfect when the data length used to calculate the correlation is finite. This results in the background noise on the reflectometry trace, and leads to a fatal limit in the dynamic range of the reflectometry. This problem is originated from the fact that the true random signal (i.e. the data to be transmitted) is used as the reference signal. To avoid this problem, some special bit-sequences such as M-sequence and Gray code are used in the random-modulation CW lidar and the complementary correlation OTDR (see N. Takeuchi, N. Sugimoto, H. Baba, and K. Sakurai, “Random modulation cw lidar,” Appl. Opt., 22, 1382 (1983); M. Nazarathy, S. A. Newton, R. P. Giffard, D. S. Moberly, F. Sischika, W. R. Trutna, Jr., and S. Foster, “Real-time long range complementary correlation optical time domain reflectometer,” J. Lightwave Technology, 7, 24 (1989); and U.S. Pat. No. 5,000,568). These special bit-sequences have a delta-function-like autocorrelation function without background noise, and they are ideal as a reference signal for cross-correlation detection. However, since the objective of the present invention is the in-service monitoring, and hence, there is no choice of the bit-sequence for the reference signal, and these methods in M. Nazarathy, S. A. Newton, R. P. Giffard, D. S. Moberly, F. Sischika, W. R. Trutna, Jr., and S. Foster, “Real-time long range complementary correlation optical time domain reflectometer,” J. Lightwave Technology, 7, 24 (1989) and U.S. Pat. No. 5,000,568 cannot be applied.